An article fabricated on a production line may be inspected for certain features, including defects that might degrade the performance of the article or a system including the article. For instance, a glass substrate for a hard disk may be inspected for certain surface features, including surface and subsurface defects that might hinder follow on production steps (e.g., washing, sputtering, polishing, and/or etching) and further degrade the performance of the disk or the hard disk drive.
In some instances, an imaging apparatus includes a camera to record images of features of a transparent article to use in performing detection, identification, and/or shape analysis of the features. For image based feature detection, the transparent article is illuminated by a light source positioned above the article while a camera records images. In such an arrangement, a majority of the light projected on the transparent article penetrates and refracts through the article while a small fraction of the light is reflected back to the camera due to the transparency of the article. Consequently, some defects may go undetected.